Small electromechanical machines are very often used in mechatronic systems. A mechatronic system is required to be flexible and typically controls several internal sub-systems. The most common example is the control of a multi-shaft motion with a variable speed. The traditional solution uses a gear train where the speed, torque and direction of rotation are determined by a single motor and by the gear rated parameters. In a mechatronic solution, each shaft is controlled by its own electronically controlled motor, giving a higher flexibility and a higher efficiency. Such mechatronic systems are found in quartz watches, mobile phones, computers, (automatically guided) vehicles, industrial robots . . .
Manufacturers of computer hardware, automotive applications, office equipment, medical equipment, instrumentation for measurement and control, robots and handling systems are the main purchasers of small motors. Especially, small permanent magnet (PM) motors are frequently used. In 2002, the world production of PM motors was estimated to be 4.68 billion units (with a value of 38.9 billion U.S. $). In a car, for example, the number of small motors can run up to one hundred. Small PM brushless motors are e.g., used in hard disk drives and cooling fans of computers. In 2002, the worldwide production of computers was estimated to be 200 million units and the production of hard disk drives was approximately 250 million units.
The working principle of electrical machines and actuators relies upon the presence of a magnetic field that depends on both time and space. The most straightforward realisation thereof is by a set of AC current-carrying coils, for which the time-dependence is obtained by the alternating current and the spatial dependence is obtained by the spatial dislocating of multiple coils. By convention, a small electrical machine or actuator has a mechanical power below 1 kW. Energy efficiency is one of the most important performance indicators of an electromechanical machine. Although today the efficiency is high for high-power machines, the efficiency decreases rapidly for lower-power machine. The lower the power range, the lower the absolute value of the power losses, which is why one becomes ignorant about the lower efficiency. However, in view of the increasing number of small machines used, even small losses become significant.
The lower the mechanical power, the lower the expected/required efficiency. The low efficiency of the small electrical machines originates mainly from the fact that the stator Joule losses relative to the mechanical power increase. The latter can for example be seen in FIG. L1 illustrating the different losses occurring in an electrical machine. The stator Joule losses are the dominating losses in the low power range. This is also acknowledged in the new machine efficiency standard, introduced in 2008, specifying the IE1, IE2, IE3 and IE4 classes. From the 1st of January 2017, all new electrical machines beyond 0.75 kW must comply to the IE3 requirement with some exceptions.
There is a need for electrical machines providing low power in an efficient way.